1) Field of the Invention
The present invention relates to a process for recycling polyester materials. Specifically it relates to the purification of crude bis hydroxyethyl terephthalate (BHET) from colored polyester (post consumer/scrap production) waste streams. More specifically, colored polyester for recycling is depolymerized by the addition of glycol to form the monomer—BHET. The BHET is contacted with activated carbon to remove some colorant. The remaining colorant is extracted with water, alcohol, or glycol to produce white, purified BHET. The purified BHET is polymerized to polyethylene terephthalate meeting food contact specifications.
2) Prior Art
Typically waste polyester has uses in areas where color is not a factor, for example, as textile insulation, furniture padding, fiber fill, carpet fibers, etc., which enable the user to recycle any color or mixture of colors of waste polyester, and apply pigments or dyes to mask any undesirable color. In this production process, the waste polyester is merely re-extruded as molten polyester, optionally using a vented extruder to remove moisture, and filtered to remove bits of metal, paper, etc. The pigment or dye may then be added and the polyester extruded as fiber or film, for example. These applications generally use 100% waste polyester.
Additionally another method of treatment of waste polyester of the quality described above is to pelletize the waste, through grinding, cutting or other means to produce a low density ‘flock’ particle, similar in shape to a polymer melt chip, a more easily handled form of the waste polyester, which can then be fed by a conventional mass flow device directly into a reaction vessel which is part of a process for making PET polymer, either in a batch mode or a continuous mode, and then adjusting the color of the final product by the addition of pigments or dyes as described above before final extrusion to fiber or chip or film, for example. Waste polyester can be from any source, such as post-consumer articles like beverage bottles, or from scrap production of polyester, either as fiber waste, polymer waste or solid waste.
It is also known to recycle waste post consumer product polyester in packaging applications, such as for beverages. These processes generally use methanolysis, hydrolysis, or glycolysis reactions to depolymerize the waste polyester to the monomer and then purify and use this monomer as the feedstock for polymerization with virgin raw materials, for example.
In a methanolysis polyethylene terephthalate (PET) process, waste PET is reacted with methanol to produce dimethyl terephthalate (DMT) and ethylene glycol (EG). These reaction products may be readily purified and used thereafter to produce virgin polyester (polyester formed from its raw materials).
Hydrolysis of PET is reacting waste PET with water to depolymerize into terephthalic acid (TA) and EG. However, it is known that certain types of contaminants generally present in waste PET are very difficult and expensive to remove from TA.
Glycolysis of PET is reacting waste PET with glycol such as EG thus producing the monomer, bis hydroxyethyl terephthalate (BHET) and other oligomers. This process has significant advantages over methanolysis or hydrolysis primarily because the BHET may be used as a raw material in either a DMT or TA polyester production process without significant major modification of the production facility.
These chemical depolymerization processes, methanolysis, hydrolysis, or glycolysis require the addition of other materials to the waste polyester, i.e., methanol, water, or glycol respectively, followed by a purification step. The total manufacturing costs of these recycle processes exceed that of manufacturing virgin PET resin.
U.S. Pat. No. 5,635,584 to Ekart et al. discloses a process whereby post consumer polyester is purified using glycolysis to depolymerize the previously used polyester materials, dissolving the depolymerized materials in a hot solvent, purification of the solution containing the depolymerized materials, and subsequent precipitation of the depolymerized materials for reuse as raw material (for polyester packaging materials). This process requires the use of a hot solvent, such as n-butyl alcohol to purify the raw materials.
U.S. Pat. No. 5,869,543 to Boos et al. discloses a method for synthesizing polyethylene terephthalate from waste polyethylene terephthalate by cleavage with ethylene glycol and separation of BHET from the cleavage mixture with a specific temperature program, recrystallization from water, and esterification and polycondensation with the addition of terephthalic acid.
U.S. Pat. Nos. 5,504,121 and 5,602,187 to West disclose a process for removing contaminants from colored PET components by depolymerizing polyethylene terephthalate by reacting ethylene glycol with the material for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or BHET; contacting the short chain PET polymers and/or BHET with activated carbon to remove dye; recovering short chain PET polymers and/or BHET and ethylene glycol; and hydrolyzing the recovered short chain PET polymers and/or BHET at elevated pressure and temperature for a predetermined period of time to form an ethylene glycol solution and crystals of terephthalic acid. The hydrolyzed short chain PET polymers and/or BHET may then be further treated with activated carbon to remove dye and purify the short chain PET polymers and/or BHET.
U.S. Pat. No. 6,642,350 to Asakawa et al. and U.S. Pat. No. 6,630,601 to Inada et al. disclose a process in which crude BHET is passed through a bed of activated carbon followed by an ion-exchange bed.
These prior art processes are either not economic or fail to purify the post consumer PET, especially colored PET, to a sufficient purity and color so that it can be used as the raw material at a reasonable, greater than about 10%, rate in a virgin PET process.